Put some butter or margarine in a coffee mug or other microwave safe bowl/cup. I use about 2 TBLS.
Microwave the butter/margarine, covered, until it's all melted. Be careful when taking the melted butter/margarine out of the microwave!
Stir in some white or brown sugar. I like brown sugar better, but white will do. Put in enough so that it's still liquidy, but that there isn't much butter floating on top.
If you want, add some vanilla. I but the cheapest imitation vanilla to keep around for moments like this.
Stir in some cocoa powder. You can make it as runny or stiff as you want.